I cant say that I fully understand your question, but here goes...
The Mann Act regulates interstate commerce and travel, not prostitution and pornography production. Interstate travel is outside the authority of the states and solely a federal issue. Prostitution and adult content production are a state issue, and even though, legal in some states, the transportation of women to those states could still be considered illegal under the act.
Think of it this way. The sale of medical marijuana is legal under California state law, not federal law. If someone was raising a crop of marijuana in New York for sale in California and was caught delivering 2 tons of marijuana in the back of their truck in Kansas as they were driving the marijuana to California, do you believe they could claim that they are innocent of possession and distribution with the intent to sell since medical marijuana is legal in California ? Not likely.
The Mann Act is not usually invoked in cases as you have presented though. The immoral aspect of the law as basically be replaced with illegal, but that doesnt mean the feds couldnt try. I think you would have to look at a case by case basis. Unfortunately there are no absolute truths in law.